Deciding on whether to mount your psu either facing up or down depends entirely on a few factors namely case design airflow with other fans and the power supply unit itself.
Bottom mounted psu on carpet.
The carpet will block the airflow to the psu and in worst cases if you have no psu fan filter installed then the small fiber particles from the carpet can get deposited inside.
This is typically at the bottom of the case in most modern chassis designs.
In my case i get cool air for the intake and the hot air goes out the side of the case.
A bottom mounted psu keeps the psu cooler not the case.
The problem of bottom mounted power.
Avoid placing case on carpet for bottom mounted psu with fan down never put your computer case on the carpet when you have a bottom mounted psu fan facing downwards.
My computer sits on my desk and imo i would never set a computer on carpet.
Determine where the psu is intended to fit.
Extra cables are easier to manage as they lay on the bottom.
Being placed at the top of the case a power supply could.
Mounting a psu in the top of the case should be ok as long as it vents to the top.
This means that they will run a bit hotter than a psu that is mounted on the bottom which draws air from outside the case.
Once you ve located where the psu will sit check the case for any grills or vents directly beneath this location.
Hot air rises and cool air you get the point.
A top mounted psu is like a 2nd exhaust fan but if you want it to run cooler go with a bottom mounted psu.
To figure out whether your psu fan needs to point up or down start by looking over your pc case.
My psu is mounted in the bottom of my case a tj07.
You have to have space under the case though and be careful on high carpet.